AURORA — Suspected gunman James Eagan Holmes planned an Aurora movie theater massacre with “calculation and deliberation,” arming himself with ammunition delivered to his home and school over a period of months.

That ammunition, Police Chief Dan Oates said Saturday, was used to kill and injure 70 people and booby-trap his apartment so as “to kill anyone who entered it.”

As the names were released of the 12 people who died after being blinded by gas and methodically mowed down Friday, President Barack Obama announced he would visit Colorado Sunday to speak to victims and their families. He vowed to bring “whoever’s responsible for this heinous crime to justice.”

Authorities, using a robot to set off a controlled detonation in Holmes’ apartment, disabled the most threatening of 30 softball-sized improvised explosive devices, rigged in such an intricate network that a veteran FBI special agent characterized it as “sophisticated” and “complex.”

Oates said the explosives found in the north Aurora apartment were directly aimed at his officers, who were likely to walk through the door first.

“This apartment was designed to kill. If you think we’re mad, we’re sure as hell angry,” Oates said during an afternoon press conference.

He would not comment on whether Holmes, who is in police custody, is cooperating with authorities. Authorities have not revealed Holmes’ possible motives.

FBI Special Agent James Yacone said evidence taken out of Holmes’ apartment will be sent to Quantico and that while the area is believed to be generally safe, “the threat has not been completely eliminated.”

Throughout Saturday, Aurora residents flooded the street corner adjacent to the theater, laying flowers, cards and photos, lighting candles and in some instances, sobbing.

Strangers hugged each other and exchanged phone numbers.

Many who stopped by didn’t know anyone in the theater that night. One card, attached to flowers, read: “with love from Dubuque Iowa.”

Aurora Mayor Steve Hogan tried to provide a unified voice for those who are suffering in his city.

“We’re still reeling,” he said. “We will do whatever it takes for as long as it takes to try and help (the victims).”

Witnesses at the Century Aurora 16 complex said Holmes, 24, slipped into the midnight premiere of “The Dark Knight Rises” through an emergency exit door, armed with three guns and wearing a ballistic helmet, gas mask and body shields. He tossed two hissing gas canisters and calmly walked up the aisle firing at moviegoers, killing 12 and wounding 58.

Afterward, police found Holmes, who until recently was a graduate student in neuroscience at the University of Colorado School of Medicine, calmly waiting for them in the parking lot.

His first court appearance will be at 9:30 a.m. Monday at the Arapahoe County Courts.

Oates said Holmes had received multiple deliveries at home and school over the past four months, which police believe included ammunition and possibly bomb-making materials.

Inside the apartment, police found multiple booby traps, along with IEDs, trip wires and accelerants with trigger mechanisms.

“What we’re seeing here is evidence of some calculation and deliberation,” he said.

Officials estimate that about 30 aerial shells were also inside the 800-square-foot apartment. Liquid inside of jars placed throughout the room is assumed to be liquid accelerants, but officials could not provide further details.

“There are still unknowns,” said Sgt. Cassidee Carlson of Aurora Police Department. “Keep in mind that we are not exactly sure of everything that’s in there.”

Details about the 12 people who died started to emerge Saturday. The victims included an aspiring sports journalist, a sailor and a Target employee who dove on his girlfriend to protect her from gunfire.

The oldest victim was 51. The youngest, Veronica Moser, was only 6 years old. A student at Holly Ridge Elementary School in Denver, the blond-haired, blue-eyed girl’s mother, Ashley Moser, suffered two gunshot wounds in the abdomen and the neck, and may be partially paralyzed.

The 25-year-old woman passes in and out of consciousness, asking about her daughter, said her aunt, Annie Dalton. She doesn’t know yet that Veronica has died.

“It’s a nightmare right now,” Dalton said.

A prayer vigil will be held 6:30 p.m. Sunday at the Aurora Municipal Center, 15151 E. Alameda Parkway in Aurora. Those who attend are asked to gather on the westside steps of the building.

Colorado Gov. John Hickenlooper stopped by several hospitals Saturday to visit the injured, and said he heard “amazing stories of people acting selflessly.”

“I talked to a family this morning and they were right in the front row. It was a girlfriend and her boyfriend and the boyfriend’s father. The father of the boyfriend lay down on her after she was shot,” Hickenlooper said. “The doctor feels the father’s action saved her life.”

In another case, a man shot in the leg managed to flee the theater. A woman who took off her belt and made a tourniquet for his leg might have saved his life, Hickenlooper said.

Christian Bale, the star of the Batman film, issued a statement saying his heart goes out to the victims and their families.

“Words cannot express the horror that I feel,” Bale said. Warner Bros., the distributor of the Batman movie, decided to withhold its box-office numbers for the weekend. Sony, Fox, Disney, Universal Paramount and Lionsgate also decided to wait on reporting revenue reports.

More in News

The California Highway Patrol has released a video of one of its helicopter crews performing a dramatic rescue of two British hikers who were stranded on the edge of a massive granite cliff overlooking Yosemite Valley as a major snow storm was nearing. The video, posted to Facebook by the CHP's Central Division Air Operations, shows a helicopter crew that...

A researcher at a university in the U.K. who came up with a mathematical equation declaring the third Monday in January as "the most depressing day of the year." A mental health advocate has debunked that theory.

While Trump's relationship with much of his base remains strong, two years after his inauguration his ties are fraying with some voters, the kind who voted in droves for Trump in 2016 in key pockets throughout the industrial Midwest and flipped previously Democratic states to him.